Digital Foundry has a real, actual 4090 that they're testing out. They put out a teaser video that has enough information to figure out that like for like (Both with DLSS 2) , the 4090 is only about 1.5-1.6x faster than 3090Ti !
https://www.youtube.com/watch?v=qyGWFI1cuZQ
Data point 1: DLSS3 is 389%, while DLSS3 is 255.4% (Both 4090) This means DLSS 2 is 0.656x of DLSS 3. Note, the baseline is 4090 with DLSS off.
Data point 2: DLSS2 3090Ti is 100%, while DLSS 3 4090 is 235.8%.
Multiplying by 0.66x (the difference b/w DLSS 2 & 3, we get 154%, which is the true performance uplift vs 3090Ti.
Needless to say, super disappointing and well below what leatherJacketMan claimed (2x - 4x uplift).